Technical Field
The present description relates to techniques for providing gamut extension in the display of color images.
One or more embodiments can find application in devices that can be used for reproducing images, for example, in projectors such as the so-called “laser pico projectors.”
Description of the Related Art
The continuous development of image-display devices (the term images is here to be understood as referring indifferently both to fixed images and to images/frames of video sequences) has led to providing devices able to display a gamut that is wider than the one provided by the input sources.
This applies, for example, when latest-generation display devices (displays and/or projectors) are used in conjunction with sources of image signals of a less advanced type (legacy images). Once again by way of example, whereas wide-gamut reproduction devices can use an adobeRGB color space, there exist projectors, for example laser projectors, which enable display of color gamuts that are even wider.
The possibility of displaying images with a certain gamut (sRGB, aRGB) with devices that are able to reproduce a wider gamut enables more saturated colors to be obtained, with the possibility of displaying an image that is more attractive to the eye. Achieving this result consequently implies “inventing” additional colors, at the same time preventing artifacts, such as banding, false colors, and hue shift in the final image.
For example, proposed in Y. Liu, et al.: “A Hue-preserving Gamut Extension Algorithm in CIELUV Color Space for Wide Gamut Displays”, 2010 3rd International Congress on Image and Signal Processing (CISP 2010), 2401-2404, is a method of gamut extension that uses cube roots, for transformations in LUV space. On the other hand, it is to be noted that the LUV space may prove deficient in terms of perceptive uniformity.
In Song Gang, et al.: “A Gamut Extension Algorithm based on RGB Space for Wide-gamut Displays”, IEEE 13th International Conference on Communication Technology (ICCT), 2011 a solution is proposed that operates in the RGB space (hence a space not perceptively uniform) that seeks to minimize the errors that derive therefrom via a boosting function calculated on the basis of the hue.
In order to prevent color artifacts, such as for example hue shift, a possible strategy may be to perform gamut extension in a color space that is perceptively uniform such as the LCh color space. Operating in the LCh color space may, however, prove burdensome from the computational standpoint.
It has been noted that it is possible to prevent recourse to transformation in the LCh color space, for example with reference to the CIExyY 1931 color space. The character of non-perceptively uniform color space extends in itself also to the CIE 1931xyY color space, rendering gamut extension in this domain more complex.
In the paper by Gang Song, et al.: “A Gamut Extension Algorithm Based on Saturation for Wide-gamut Displays”, International Conference on Multimedia Technology, vol. 12, 2010, 493-495, it is proposed to implement the process of gamut extension in the xyY domain moving along straight lines. This procedure entails, however, the risk of generating hue-shift artifacts when the operation of extension or boosting is carried out.
The criticality of these artifacts is acknowledged, for example, in L. A. Taplin et al.: “When Good Hues Go Bad”, Munsell Color Science Laboratory Center for Imaging Science, Rochester Institute of Technology, Rochester, N.Y.